Plasma confinement by use of preferred RF return path

ABSTRACT

A confinement assembly for confining a discharge within an interaction space of a plasma processing apparatus comprising a stack of rings and at least one electrically conductive member. The rings are spaced apart from each other to form slots therebetween and are positioned to surround the interaction space. At least one electrically conductive member electrically couples each ring. The electrically conductive member contacts each ring at least at a point inside of the outer circumference of each ring.

CROSS-RELATED APPLICATIONS

The present application is a divisional application based on applicationSer. No. 09/846,172, filed Apr. 30, 2001 now U.S. Pat. No. 6,202,381,entitled “Plasma confinement by use of preferred return path” in thename of inventor Eric H. Lenz commonly assigned herewith.

FIELD OF THE INVENTION

The present invention relates to plasma etching apparatus. Moreparticularly, the present invention relates to improved techniques forcontrolling plasma formation in a plasma processing chamber.

BACKGROUND OF THE INVENTION

The use of plasma-enhanced processes in the manufacture ofsemiconductor-based products (such as integrated circuits or flat paneldisplays) is well known. Generally speaking, plasma-enhanced processesinvolve the processing of a substrate (e.g., a glass panel or asemiconductor wafer) in a plasma processing chamber. Within the plasmaprocessing chamber, a plasma may be formed out of appropriate etchant ordeposition source gases to respectively etch or deposit a layer ofmaterial on the surface of the substrate.

FIG. 1 depicts a capacitively-coupled plasma processing chamber 100,representing an exemplary plasma processing chamber of the typestypically employed to etch a substrate. A chuck 104 represents theworkpiece holder on which a substrate 106 is positioned during etching.The chuck 104 may be implemented by any suitable chucking technique,e.g., electrostatic, mechanical, clamping, vacuum, or the like. Duringetching, the chuck 104 is typically supplied with RF power having afrequency of, for example, about 400 Khz to about 27 Mhz, by an RF powersupply 110. In some systems, chuck 104 may be supplied with dualfrequencies, e.g., 2 MHz and 27 MHz simultaneously during etching.

A reactor top 112, formed of a conductive material such as aluminum, isdisposed above substrate 106. Confinement rings 102 may be coupled in afixed manner to reactor top 112 or may be coupled to cam-based plungers(not shown in FIG. 1) that allow confinement rings 102 to be raised andlowered without moving reactor top 112.

In general, confinement rings 102 help confine the etching plasma to theregion above substrate 106 to improve process control and to ensurerepeatability. Although only two confinement rings are shown in theexample of FIG. 1, it should be understood that any number ofconfinement rings may be provided.

An upper electrode 114 and a baffle 116 are also coupled to reactor top112. The upper electrode 114 may be grounded (as in the case of FIG. 1)or may be powered by another RF power source 120 during etching. If theupper electrode 114 is powered, it may be insulated from the remainderof the reactor to isolate the electrode from ground. During etching,plasma is formed from etchant source gas supplied via a gas line 122 andthe baffle 116.

When RF power is supplied to the chuck 104 (from the radio frequencygenerator 110), equipotential field lines are set up over the substrate106. During plasma processing, the positive ions accelerate across theequipotential field lines to impinge on the surface of substrate 106,thereby providing the desired etch effect (such as improving etchdirectionality). Due to geometry factors, however, the field lines maynot be uniform across the substrate surface and may vary significantlyat the edge of substrate 106. Accordingly, a focus ring is typicallyprovided to improve process uniformity across the entire substratesurface. With reference to FIG. 1, chuck 104 is shown disposed within afocus ring 108, which is typically formed of a suitable dielectricmaterial such as ceramic, quartz, or plastic.

The equipotential field lines that are set up during plasma etching maybe seen more clearly in FIG. 1B. In FIG. 1B, the presence of focus ring108 allows the equipotential field lines to be disposed substantiallyuniformly over the entire surface of the substrate, thereby allowingetching to proceed in a uniform manner across the substrate. As seen byFIG. 2, however, some of the equipotential field lines also extend intothe region 160 outside of focus ring 108. The presence of theequipotential field lines in region 160 may cause any charged particlesthat leak past the confinement rings to accelerate in a directionperpendicular to the equipotential field lines toward the chamber walls.This acceleration and the subsequent collision between the chargedparticles and the chamber walls may generate secondary electrons, whichmay ignite and/or sustain unconfined plasma in the region 160 (i.e.,unintended plasma that is not confined to region directly above thesubstrate).

Furthermore, current return paths have relied on the chamber wall 118for a return path or a return path outside the chamber. Magnetic fieldsare generated from the return paths and cause magnetic fields that canlight and sustain a plasma outside the confined region. The dotted linesin FIGS. 1A and 1B illustrate the current return path along the chamberwall 118.

The inadvertent generation of plasma in the region 160 renders the etchprocess difficult to control and may damage components within thisregion. By way of example, this unconfined plasma, which may beunplanned and/or intermittent, changes the location of power absorbed bythe plasma within the plasma processing chamber, thereby making itdifficult to control the delivery of power to the chuck to achieveconsistent, repeatable etch results. As another example, the presence ofunwanted plasma in region 160 may cause damage to the chamber door (notshown), particularly to the seals that are provided therewith. Thechamber door is necessary for substrate transport into and out of thechamber, and if the seals are damaged, accurate control of the chamberpressure may be difficult. When the seals and/or other components in theregion 160 are inadvertently attacked by the plasma, particulate and/orpolymeric contaminants may form along the chamber walls, potentiallyleading to contamination of the etch environment.

Accordingly, it would be desirable to provide techniques for minimizingand/or eliminating the unwanted plasma formation in the region outsideof the focus ring of the plasma processing chamber.

BRIEF DESCRIPTION OF THE INVENTION

A confinement assembly for confining a discharge within an interactionspace of a plasma processing apparatus comprising a stack of rings andat least one electrically conductive member. The rings are spaced apartfrom each other to form slots therebetween and are positioned tosurround the interaction space. At least one electrically conductivemember electrically couples each ring. The electrically conductivemember contacts each ring at least at a point inside of the outercircumference of each ring.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of this specification, illustrate one or more embodiments of thepresent invention and, together with the detailed description, serve toexplain the principles and implementations of the invention.

In the drawings:

FIG. 1A is schematic diagram illustrating a typical capacitively coupledplasma processing chamber in accordance with the prior art;

FIG. 1B a schematic diagram illustrating the equipotential field linesthat may be formed in the plasma processing chamber of FIG. 1A duringplasma processing in accordance with the prior art;

FIG. 2 is a schematic cross-sectional diagram of a capacitively coupledplasma processing chamber having a preferred return path in accordancewith a specific embodiment of the present invention;

FIG. 3 illustrates a top view of a confinement ring having a preferredreturn path in accordance with a specific embodiment of the presentinvention; and

FIG. 4 is a flow chart diagram illustrating a method for forming aconfinement assembly of a plasma processing chamber.

DETAILED DESCRIPTION

Embodiments of the present invention are described herein in the contextof controlling plasma formation in a plasma processing chamber. Those ofordinary skill in the art will realize that the following detaileddescription of the present invention is illustrative only and is notintended to be in any way limiting. Other embodiments of the presentinvention will readily suggest themselves to such skilled persons havingthe benefit of this disclosure. Reference will now be made in detail toimplementations of the present invention as illustrated in theaccompanying drawings. The same reference indicators will be usedthroughout the drawings and the following detailed description to referto the same or like parts.

In the interest of clarity, not all of the routine features of theimplementations described herein are shown and described. It will, ofcourse, be appreciated that in the development of any such actualimplementation, numerous implementation-specific decisions must be madein order to achieve the developer's specific goals, such as compliancewith application- and business-related constraints, and that thesespecific goals will vary from one implementation to another and from onedeveloper to another. Moreover, it will be appreciated that such adevelopment effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking of engineering for those ofordinary skill in the art having the benefit of this disclosure.

In accordance with one aspect of the present invention, process controlis substantially improved by reducing or eliminating the unconfinedplasma (i.e., the unwanted plasma that is inadvertently ignited and/orsustained outside of the focus ring and the walls of the plasmachamber). Confinement rings consists of a stack of rings spaced apartfrom each other to form slots therebetween and positioned to surroundthe interaction space. During operation of the plasma processingapparatus, the distance an exiting charged particle must travel in theslot is substantially longer than its mean free path. As the term isemployed herein, the region outside of the focus ring refers to annularregion of the plasma processing chamber which is external to the columnof space whose outer periphery is defined by the circumference of thefocus ring. The plasma is preferably confirmed within this column ofspace. Outside of the focus ring, the electric field is preferablyreduced to the point where plasma can no longer be sustained. Byeliminating the unconfined plasma, the amount of power absorbed by theetching plasma that is disposed above the substrate may be moreconsistent from 1, substrate to substrate, thereby rendering the etchrepeatable. The elimination of the unconfined plasma also helps reducethe corrosion or break down of components disposed in the region outsideof the focus ring (e.g., door seals).

In accordance with one embodiment of the present invention, there isprovided a confinement assembly, including a stack of focus ringsconfigured to concentrate the equipotential field lines in the focusring body. The focus rings preferably include at least one cavitydisposed evenly around each ring and a corresponding number ofelectrically conductive members for returning the current to the ground.Each cavity receives and accommodates one member. It is believed thatthis configuration substantially reduces the density of equipotentialfield lines in the region outside of the focus ring. By substantiallyreducing the density of equipotential field lines in the near-vacuumregion outside of the focus ring, the amount of energy acquired by anycharged particle that leaks into this area is substantially reduced,thereby essentially eliminating the possibility of plasma formationand/or sustenance in this region.

Other embodiments may exist in which an electrically conductive memberconnects each ring at least a point inside of the outer circumference ofthe ring. The magnetic fields generated by the electrically conductivemember are substantially reduced from an excluded region when the memberis disposed within the ring. The member may include any shape or formthat allows the current to return to the ground while substantiallyreducing the magnetic fields generated by the member. Examples ofmembers may be rods, strings, or beams connecting the rings at least ata point inside of the outer circumference of the rings.

The confinement rings 216 also include at least one cavity 302,equidistant from each other, as illustrated in FIG. 3. FIG. 3illustrates a preferred embodiment in which the confinement ringsinclude three cavities. The cavities 302 may include a slit. Each slitreceives an electrically conductive member 218 that may not contact theconfinement rings 216. The slits are preferably positioned equidistantlybetween the inner edge and the outer edge of the confinement rings 216as illustrated in FIG. 3 to deeply bury and isolate the magnetic fieldsgenerated by the members 218. Such members 218 may preferably include abeam of highly conductive material, such as aluminum. As seen in FIG. 2,each member 218 may be disposed vertically within the cavities 302 ofeach confinement ring 216. The top of each member 218 electricallycontacts the upper electrode 204. The bottom of each member 216electrically contacts the grounded shield 219. Therefore, each member216 is sandwiched between the upper electrode 204 and the groundedshield 219 which also provides support to each member 218.

The power supply 212 provides an RF current to the lower electrode 206.During the process, the current travels towards the upper electrode 204through the confinement region 214. Because the upper electrode 204electrically contacts members 218, the current travels through thispreferred path, i.e. members 218, towards the grounded shield 219.

The presence of the members 218 embedded within the cavities 302 of theconfinement rings 216 provides a preferred return path thatsubstantially reduces the density of the magnetic fields in the upperportion of annular region 220, i.e., the region outside of the focus andconfinement rings 216. The confinement rings 216 isolate the plasma andthe chamber 202 from the preferred return paths. Being embedded in theconfinement rings 216, the stray magnetic fields are kept frompenetrating into the volume between the outer edge of the confinementrings 216 and the chamber 202, i.e. the region 220. The plasma staysconfined and since the return paths are in the confinement ring area,the plasma cannot damage the return path materials. Therefore the straymagnetic fields are substantially reduced outside the confinement rings216.

Other embodiments may exist in which an electrically conductive memberconnects each ring at least a point inside of the outer circumference ofthe ring. The magnetic fields generated by the electrically conductivemember are substantially reduced from an excluded region when the memberis disposed within the ring. The member may include any shape or formthat allows the current to return to the ground while substantiallyreducing the magnetic fields generated by the member. Examples ofmembers may be rods, strings, or beams connecting the rings at a least apoint inside of the outer circumference of the rings.

FIG. 4 is a flow chart diagram illustrating a method for forming aconfinement assembly of a plasma processing chamber. At 402, a chuck forsupporting the substrate during plasma processing is provided. At 404, afocus ring is positioned to substantially encircle the wafer. At 406, astack of confinement rings, made of electrically insulating material,spaced apart from each other to form slots therebetween is positioned tosurround an interaction space defined between a top electrode and thechuck. Each confinement ring has at least one cavity formed on thesurface thereof. At 408, at least one electrically conductive memberpasses through each of the cavity of the stack of confinement rings. At410, the top of the electrically conductive member is electricallycoupled to the top electrode and the bottom of the electricallyconductive member is grounded.

While embodiments and applications of this invention have been shown anddescribed, it would be apparent to those skilled in the art having thebenefit of this disclosure that many more modifications than mentionedabove are possible without departing from the inventive concepts herein.The invention, therefore, is not to be restricted except in the spiritof the appended claims.

What is claimed is:
 1. A method for forming a confinement assembly of aplasma processing chamber having an interaction space confined between atop electrode and a bottom electrode, said method comprising:positioning a stack of confinement rings, made of electricallyinsulating material, spaced apart from each other to form slotstherebetween to surround the interaction space of the plasma processingchamber, each confinement ring having at least one cavity formed on thesurface thereof; positioning a focus ring to surround the bottomelectrode; and passing an electrically conductive member through each ofsaid cavity of said stack of confinement rings, a top of said conductivemember electrically coupled to the top electrode, said electricallyconductive member grounded at a bottom thereof.
 2. The method accordingto claim 1 wherein said stack of rings comprises an electricallyinsulating material.
 3. The method according to claim 2 wherein saidelectrically insulating material comprises quartz.
 4. The methodaccording to claim 1 wherein said electrically conducive membercompressing aluminum.
 5. A method for confining plasma in a plasmaprocessing chamber, the method comprising: providing a chuck forsupporting the substrate during plasma processing; encircling said chunkwith a focus ring; positioning a stack of confinement rings, made ofelectrically insulating material, spaced apart from each other to formslots therebetween, said stack of confinement rings surrounding aninteraction space defined between the top electrode and said chunk, eachconfinement ring having at least one cavity formed on the surfacethereof; and passing at least one electrically conductive member througheach of said cavity of said stack of confinement rings, a top of said atleast one electrically conductive member electrically coupled to the topelectrode, said electrically conductive member grounded at a bottomthereof.
 6. The method according to claim 5 wherein said stack of ringscomprises an electrically insulating material.
 7. The method accordingto claim 6 wherein said electrically insulating material comprisesquartz.
 8. The method according to claim 5 wherein said electricallyconductive member comprises aluminum.